1. Field of The Invention
This invention relates to optical elements for radiation which are particularly useful for X-ray spectroscopy, neutron spectroscopy and the like spectroscopic analyses and which are made of artificial graphite.
2. Description of The Prior Art
As is known in the art, optical elements which are usually used in optical instruments for X-rays such as X-ray spectroscopes, X-ray microscopes and the like make use of the Bragg reflection of crystals although, in a specific case, total reflection of X-rays passing very closely to a reflection surface may be utilized. The crystals used for the above purpose should have a complete crystal structure and should be obtained in a desired size. Moreover, such crystals should have a small absorption coefficient for X-rays and should have appropriate flexibility when applied to curved spectroscopes.
One type of crystal which satisfies the above requirements is graphite. Graphite has a small absorption coefficient against X-rays and has been often employed as an optical element for X-rays. However, a single crystal of natural graphite with a large area cannot be obtained. Accordingly, it is usual to artificially obtain graphite crystals by hot processing of a pyrolytic deposit of hydrocarbon. For instance, there is known as artificial graphite compression annealed pyrographite (CAPC) or high-oriented pyrographite (HOPG) commercially sold from Union Carbide Inc. These graphite products are produced by pyrolyzing a gaseous hydrocarbon at a temperature of about 1000.degree. C. to obtain graphite crystals and annealing the crystals at 3600.degree. C. over a long time of, say, several weeks under pressure.
As is known, the Bragg equation is expressed as 2d sin .theta.=.lambda. where d is the distance between the successive lattice planes, .theta. is a reflection angle, and .lambda. is a wavelength of the reflected X-ray. It is stated that with the graphite of Union Carbide Inc., where a monochromatic X-ray, e.g. a K .alpha. line of Cu radiation (.lambda.=1.5418 .ANG.), reflects on the (002) plane, the distance between the lattice planes, d, is close to the distance between the single crystals of the graphite, i.e. d=3.354 .ANG.. The half-value width of the reflection line, .DELTA..delta..sub.002, is stated to be about 0.7.degree..
However, these artificial graphite products involve the problem that annealing under very high temperature and long time conditions at a high pressure is required as mentioned above, thus the production process being complicated with high production costs.
For focussing X-rays, it is usual to appropriately bend a single crystal plate of silicon or to form a curved lens of graphite by machining. These are also complicated in the fabrication with high production costs.
For the purpose of providing artificial graphite sheets with a large area which are simply fabricated without resorting to complicated procedures such as compression annealing and are thus inexpensive but which have complete crystallinity and good flexibility, we proposed in U.S. Pat. No. 4,788,703 (corresponding European Patent Laid-open Application No. 219,345) graphitization of a polyphenylene oxadiazole (POD) by treatment at 2800.degree. C. or higher. The graphitized sheet was flexible and was found to be suitable as a radiation optical element such as for X-rays.
The graphitized POD obtained by treating starting POD at a normal pressure at temperatures not lower than 2800.degree. C. has the following physical properties.
(1) Reflection lines against CuK .alpha. (1.5418 .ANG.) are those corresponding only to (002), (004) and (006) planes.
(2) The reflection angle (2.theta.) at the (002) plane is 26,576.degree. and the distance between the lattice planes, d, is 3,354 .ANG., which were in coincidence with those of the single crystal of graphite.
(3) The half-value widths of the reflection line (having a center at 2.theta.=26.576.degree.) at the (002) plane were, respectively, 2.0.degree. and 0.14.degree. for thermal treatment at temperatures of 2800.degree. C. and 3000.degree. C.
(4) The graphitized POD had flexibility and the area or size of the product could be increased as desired depending upon the area or size of a starting POD sheet and the size of a thermal treatment furnace.
The radiation optical element using the graphitized POD sheet exhibit good characteristics when applied as an X-ray lens, an X-ray monochromater or an optical element for neutron spectroscopy. However, the graphitized POD has the problem that a rocking characteristic which is the most important characteristic when it is applied as a radiation optical element is not satisfactory. For instance, the rocking characteristic of a graphitized POD product treated at 3000.degree. C. is 6.9.degree., which is unsatisfactory for use as a radiation optical element although such a characteristic may be further improved by hot pressing.
Moreover, the graphitized POD is also unsatisfactory with respect to the reflectivity of radiation. The radiation reflectivity has the relation with the crystallinity along the c axis of graphite. It is known that if the crystallinity along the c axis is too good, an X-ray once passing into the crystal will suffer internal reflection, causing a total reflectivity to be lowered. In order to realize a good reflection efficiency, the crystallinity along the c axis of graphite should not be too good or too bad but is required to have an appropriate value. The graphitized POD is good in crystallinity along the c axis, which adversely influences the reflection efficiency on radiation.
In recent years, X-rays or so-called soft X-rays having a wavelength of from approximately 9 or 10 .ANG.to several hundreds angstroms are being utilized in the field of lithography of semiconductor, on which industrial importance is placed. Among soft X-ray elements dealing with an X-ray having about 10 angstroms to several hundreds angstroms, those elements using diffraction should have a lattice distance, contributing to the diffraction, which corresponds to an intended wavelength, say approximately 10 angstroms to several hundreds angstroms. With graphite currently used as the X-ray element, the lattice distance contributing to the diffraction is 3.354 .ANG.. Accordingly, such graphite cannot be used for the purpose of reflection of soft X-rays.